1. Field of the Invention
The present invention relates to joints formed of selectively etched silicon wafer material. More particularly, this invention pertains to such a joint, its method of manufacture and a pendulum incorporating at least one such joint and produced by micromechanical single-crystal etching.
2. Description of the Prior Art
Numerous applications, such as flexible suspensions for pendulums, require bending spring joints having a precisely located fulcrum and a well-defined spring constant. Such bending spring joints have been produced in the past from flat metal leaf springs formed from cold-rolled metal foils. The foils are welded by a laser welding process to form a composite layer structure to attain the required accuracy with regard to fulcrum and spring constant. Another technique has been to employ a milling process to form the bending spring element from metal strip material that is etched to the desired thickness by an electrolytic polishing process.
Production processes as described above are both complex and expensive. In the composite technique, difficulties arise from the multiple layer structure of the metal spring. Furthermore, metal is generally of insufficient flexibility. Thus, the yielding point is often reached by the spring. In addition, the hysteresis that exists in metals impairs the stability of spring pretension.
Bending spring elements have also been produced in precision engineering and micromechanics from silicon single-crystal wafers by anisotropic etching and the etching resist technique. In most cases (e.g. pendulums) a strip of constant but freely selectable width, length and depth is etched into the wafer material transverse to the pendulum direction. The width, length and depth of the etching strip determine the accuracy of the location of the fulcrum of the pendulum and, thus, the pendulum length, the spring constant of the flexible pendulum joint and the transverse axis rigidity of the pendulum. The etching strip of such a bending spring joint may, as a result of the position of the crystal axes, represent, in cross section, for example a trapezoidal valley, the bottom of which runs parallel to the rear wafer area. Such a prior art joint is illustrated in FIG. 1 of the drawings.
This prior art leaves something to be desired for high precision pendulum suspensions in the case of both metal springs and bending springs produced by micromechanical etching. While an etched spring does not encounter the hysteresis problems of metal springs, the position of the pendulum fulcrum in the case of bending spring elements formed of silicon single crystal material is often not defineable with sufficient accuracy and the transverse axis rigidity is insufficient. As a result, pendulum joint resonances are too low.